This invention relates to a heat storage material that reversibly absorbs or releases heat through liquid/solid phase changes. Specifically, the invention relates to a material composed of lithium nitrate trihydrate (LiNO3.3H2O), or some non-stoichiometric mixture of LiNO3 and H2O, and containing additives to decrease the undercooling of the liquid phase.
Lithium nitrate trihydrate is of great practical interest as a heat storage material due to its large specific heat of fusion (287 J/g; 452 MJ/m3solid; 409 MJ/m3liquid), which is even larger than that of water on a volumetric basis (FIG. 1). This large volumetric energy density is useful for small form factor thermal energy storage modules, where space is a limiting factor. Furthermore, the melting temperature (Tm) of lithium nitrate trihydrate (30.1° C.) is appropriate for a wide variety of near-room temperature applications (e.g., electronics, avionics, transportation, aerospace, etc.). Therefore, lithium nitrate trihydrate is a desirable material to store thermal energy and regulate temperature.
It is commonly known that many salt hydrates do not spontaneously crystallize at the same temperature at which they melt (FIG. 2). This temperature difference is referred to as undercooling (ΔT), and it is caused by the energy barrier that must be overcome to form a crystalline nucleus large enough that it will continue to grow spontaneously (homogeneous nucleation). Undercooling degrades the performance of a thermal energy storage (TES) component by: 1) decreasing the temperature at which heat can be released and 2) decreasing the exergetic efficiency of the thermal energy storage module. Furthermore, crystallization occurs at inconsistent temperatures, and after an uncertain period of time, as homogeneous nucleation is a stochastic process, leads to unpredictable behavior of TES components. Finally, when nucleation does occur, crystallization proceeds in rapid, uncontrollable fashion, which can lead to damage of TES components.
Nucleation agents are additives having a different solid phase as compared to the TES component that promote heterogeneous nucleation (nucleation of the desired phase along the solid-liquid interface) thereby decreasing undercooling (FIG. 2, FIG. 3). Such additives have been previously demonstrated in a number of systems, most notably in the water-ice system (Vonnegut, B. and Chessin, H., Science, 1971, 174, 954-946.; U.S. Pat. No. 3,877,642). In these cases, it has been pointed out that crystal lattice similarities between the nucleation agent and the solid nucleant phase and surface energy considerations are instrumental in finding an effective nucleation catalyst (Mondalfo. L. F., Science, 1972, 176, 695.).
Previous research has identified potential nucleation agents for lithium nitrate trihydrate, including zinc hydroxy nitrate, Zn3(NO3)2(OH)4 (Hoover, M. J. et al., NASA Technical Report 19720007297, NASA-CR-150724, 1972; U.S. Pat. Pub. 2005/0167633). However, these nucleation agents have the disadvantage of allowing for a large degree of undercooling or having questionable stability over large numbers of cycles.